Field
The present disclosure relates to an organic light emitting display device in which a color filter and a top metal of a driving thin film transistor (TFT) are formed to not overlap each other in a structure where a plurality of the color filters are formed on a lower substrate, thereby preventing short-circuit between the driving TFT and a storage capacitor.
Discussion of the Related Art
With the spread of various portable electronic devices such as mobile terminals and notebook computers, the demand for display devices applied to portable electronic devices is increasing.
Examples of display devices include liquid crystal display (LCD) devices, plasma display panels (PDPs), field emission display (FED) devices, organic light emitting display devices, and others.
Display devices are easily manufactured due to advances in manufacturing technology and have benefits that include low power consumption, a high-quality image, and a large screen. Thus, the diversity of applications of these display devices is expanding.
Among such display devices, organic light emitting display devices self-emit light from an organic light emitting diode (OLED), and thus have a broader viewing angle, a better contrast, and better luminance than LCD devices. Also, it is easy to lighten and thin the organic light emitting display device. In addition, the organic light emitting display devices have a fast response time and low power consumption, and thus are attracting much attention as next-generation display devices.
Hereinafter, among various display devices, an organic light emitting display device will be described as an example.
FIG. 1 is a view schematically illustrating a pixel structure of a related art organic light emitting display device, and FIG. 2 is a cross-sectional view taken along line A1-A2 of FIG. 1 and a cross-sectional view taken along line A3-A4 of FIG. 1. In FIGS. 1 and 2, one green pixel among a plurality of red, green, and blue pixels is illustrated as an example.
Referring to FIGS. 1 and 2, a plurality of pixels are formed in a matrix type on a display panel, and one unit pixel is composed of red (R), green (G), and blue (B) sub-pixels (sub-pixels of three colors) or R, G, B, and white (W) sub-pixels (sub-pixels of four colors).
Each of the sub-pixels includes a driving circuit that drives the white OLED. The driving circuit includes a driving TFT 10 and a storage capacitor 20, which control emission of light from the white OLED, and a plurality of switching TFTs (not shown).
The driving TFT 10 and the plurality of switching TFTs (not shown) are formed in a dual-gate structure. In FIG. 2, a drain of the driving TFT 10 and the storage capacitor 20 are illustrated, but an active layer and a source electrode are not illustrated.
An overcoat layer for planarizing a substrate is formed on the driving TFT 10, the storage capacitor 20, and the color filter 30, and a bank defining an emission area is formed on the overcoat layer. An anode electrode 50 and the white OLED (not shown) are formed in the emission area of each sub-pixel, and a cathode electrode (not shown) is formed all over the substrate.
The white OLED emits white light from an organic material layer. In order to display a full-color image, a red color filter, a green color filter, and a blue color filter 30 are formed in each sub-pixel of a lower substrate (a TFT array substrate).
Here, a pigment of the color filter 30 contains a conductive material, and thus, an electrical resistance of the color filter 30 is low. Particularly, a pigment of the green color filter is a copper-containing compound (Cu-Phthalocyanine), and thus, a permittivity of the green color filter is high, causing a leakage current to electrically flow.
The drain of the driving TFT 10 and the storage capacitor 20 are formed adjacent to the color filter 30, a top metal 12 of the drain and a top metal 22 of the storage capacitor 20 are formed to overlap the color filter 30.
Therefore, the drain of the driving TFT 10 is short-circuited with the storage capacitor 20 by the green color filter.
FIG. 3 is a circuit diagram for describing a problem that the drain of the driving TFT 10 is short-circuited with the storage capacitor 20 by the color filter.
Referring to FIG. 3, even when the driving TFT 10 is floated, a leakage current flows from the storage capacitor 20 to the driving TFT 10 due to the color filter 30. For this reason, the OLED of the green sub-pixel emits light even during a non-emission period.
In an experiment in which the leakage current of the driving TFT 10 and the storage capacitor 20 caused by the color filter was measured, it was determined that the OLED excessively emits light due to the leakage current when a resistance of the color filter is equal to or less than 1 TΩ.
When the leakage current flows due to the green color filter having a low resistance, a gate-source voltage Vgs of the driving TFT 10 of the green sub-pixel is changed to 0 V with time. Under a condition in which a threshold voltage (Vth) of the driving TFT 10 is equal to or less than 0 V, when the gate-source voltage Vgs of the driving TFT 10 is 0 V, a current of several hundreds nA flows, and due to this, the OLED excessively emits light at luminance of several Nit or more.